Gear pumps are positive displacement pumps utilizing a set of gears as the displacement device. The most common type of gear used in fluid pumps is the helical gear. Helical gears are preferred because of their quiet operation, hydraulic efficiency, conjugate mechanical motion, constant sealing of the surfaces between the mating gear flanks and the minimum entrapment of fluid between the teeth that are in contact. Further, the helical gear, with its angled teeth, typically has a higher load carrying capacity than the spur gear, which has straight teeth. Because helical gears tend to run more smoothly than spur gears, helical gears can normally operate at faster speeds.
However, one substantial drawback with the helical gear is the creation of an axial thrust which results from the helical lead or angle of the gear teeth. In operation, the axial thrust must be absorbed by the bearings that support the gear shafts. As a result, relatively expensive bearings are required to absorb these axial forces.
In order to eliminate the axial thrust associated with the use of helical gears, herringbone gears were developed. A herringbone gear is constructed of adjacent helical gear halves whereby the teeth of the adjacent halves are angled in an opposite direction. A comparison of two meshing helical gear teeth 11, 12 and two meshing herringbone gear teeth 13, 14 is provided in FIGS. 1 and 2 respectively. The helical gears 11, 12 shown in FIG. 1 have one helical slant or angle with respect to the longitudinal gear axis although the helical slant of the gear 11 is angled upward from left to right in FIG. 1 and, in contrast, the helical slant of the gear 12 is angled downward from left to right in FIG. 1. In short, the magnitude of the helical slants of the gears 11, 12 are the same, but in opposite directions.
In contrast, the gears 13, 14 each have two adjacent halves with helical slants of the same magnitude but in opposing directions. The gear 13 includes adjacent gear halves 15, 16. The helical slant of the gear half 15 extends downward from left to right and the helical slant of the gear half 16 extends upward from left to right. Similarly, the gear 14 includes gear halves 17, 18. The gear half 17 has a helical slant which extends upward from left to right in FIG. 2 while the gear half 18 has a helical slant that extends downward from left to right in FIG. 2. The magnitude of the helical slants of the gear halves 15, 16, 17, 18 are the same or substantially the same but the gear halves disposed at the left in FIG. 2, specifically the gear halves 15, 17 have helical slants that are oppositely directed to the gear halves disposed at the right in FIG. 2, specifically the gear halves 16, 18.
By incorporating gear halves such as 15, 16 and 17, 18 which have helical slants of the same magnitude but in opposite directions, the herringbone gears 13, 14 provide excellent power or fluid transmission without an axial thrust which is the result of the helical slant of the helical gears 11, 12 as shown in FIG. 1.
It will also be noted that the gear halves 15, 16 and 17, 18 are disposed on opposite sides of a central plane shown at the line 19 drawn in phantom which passes through each gear 13, 14 perpendicular to the longitudinal axes 21, 22 of the gears 13, 14 and at a mid-point along the longitudinal axes 21, 22 of the gears 13, 14.
However, while effective in eliminating the axial thrust imposed upon the shaft and bearing by helical gears, herringbone gears are difficult and expensive to manufacture. Specifically, they cannot be manufactured using a hobbing or shaping process due to the change in direction of the helical slant. As a result, the manufacture of herringbone gears in the past has been labor intensive and, as a result, expensive.
One approach at alleviating this problem has been to manufacture "pseudo" herringbone gears. Pseudo herringbone gears include two oppositely angled helical gears that are coupled or attached together at the longitudinally central plane such as 19 shown in FIG. 2. One such method for manufacturing pseudo herringbone gears is disclosed in U.S. Pat. No. 4,690,009. However, the process disclosed in the '009 patent is relatively slow and requires complex machine tools to carry out the operation.
Currently, there is no available manufacturing process to manufacture herringbone gears using a molding method or an injection molding method. Such a method would be beneficial due to its high speed of operation and low cost. A molding process would also result in a unitary gear instead of two gears that are coupled or attached together. Accordingly, there is a need in the pumping and pump manufacture industry for a method of manufacturing herringbone gears which utilizes a molding process.